The present invention relates to a method for coating various liquid compositions (herein referred to as coating solutions) onto continuously running support webs for use in the manufacture of photographic film materials, photographic printing paper, magnetic recording materials such as magnetic recording tape, adhesive tape, information recording paper such as pressure-sensitive paper or thermal paper, and materials for use in photomechanical processes. More particularly, the present invention relates to a curtain coating method for applying coating solutions onto webs.
There are various coating methods known in which a thin film of coating solution is allowed to impinge against a running web. Curtain coating is a typical one of such methods. In the curtain coating method, a free-falling curtain formed from one or more coating solutions is allowed to impinge against an object of interest, whereby a coating film is formed on that object. The curtain coating method has long been used in coating furniture, iron plates, etc., but, in recent years, as described in U.S. Pat. Nos. 3,508,947 and 3,632,374, attempts have been made to apply the curtain coating method to areas such as the manufacture of photographic materials where particularly high precision is required.
In the curtain coating method, it is very important that the free-falling curtain be applied uniformly at the time when the coating operation is started (herein referred to as "the application time"). Compared to a bead coating method using a slide hopper, the curtain coating method requires faster application, and the volume of the coating solution to be fed increases accordingly. This presents difficulty in achieving uniform coating at application time. If uniform application is not achievable, the coating solution will scatter to foul the surrounding area, or an undesirably thick coating will remain partly wet even after the passage through the drying zone, eventually fouling the transport rollers. Both of these phenomena lead to defective final products.
A typical example of the methods that have been proposed for achieving uniform application in the practice of curtain coating is described in U.S. Pat. No. 3,508,947. In this example, a rotatable or slidable device called a "deflector" (herein sometimes referred to as an "applicator plate") is used to form a stable curtain that insures the coating solution is supplied at a predetermined rate during the application time and to recover the coating solution prior to application.
FIG. 1 is a schematic side view, partly in section, illustrating the coating method described in U.S. Pat. No. 3,508,947. A coating solution 1 flowing over the sliding surface 5 of a slide hopper 2 falls freely in the form of thin film down the distal end of the sliding surface 5, thereby forming a curtain 6, which impinges against a running web 8 to form a coating thereon.
Prior to the start of application, a rectangular flat applicator plate 4 is extended into the falling curtain 6, as indicated by a dashed line, so that the coating solution flows down the applicator plate 4 and is then collected in a recovery tank 10. At the application time, the plate 4 pivots about a fulcrum 7 to be retracted to the position indicated by a solid line, and the coating solution is then applied onto the web 8 by permitting the curtain 6 to fall on the web. Both side edges of the curtain 6 are held by edge guides 3 that extend from the distal end of the sliding surface 5 to a point below the position where the curtain 6 impinges against the web 8.
However, the above-described method in which the curtain 6 that flows down prior to the application time is received by the applicator plate 4, which is rotatably retracted at the application time to come out of engagement with the curtain 6 suffers the disadvantage that at the moment the entire portion of the coating solution is applied across the entire width of the web 8, an undesirably thick coating forms in a certain area of the web 8. That is, at the moment the rotatably retracted applicator plate 4 comes out of engagement with the curtain 6 at the start of application, the coating solution is applied all at once across the entire width of the web 8, thereby forming an undesirably thick coating in a certain area of the web.
The cause of the formation of an undesirably thick coating may be explained as follows: When the curtain 6 impinges against the applicator plate 4 held in the position where it is extended into the curtain 6, a liquid mass H (called a "heel") collects upstream of the point of impingement, as shown in FIG. 2, and the curtain 6 is transferred from the applicator plate 4 onto the web 8 accompanied by the heel H. (Details of the heel formation were reported by S. F. Kistler and L. E. Scriven at the AIChE Winter Meeting in 1982.)
The present invention further relates to a coating method for use in the manufacture of photographic materials such as photographic films and print paper, materials for use in photomechanical processes, magnetic recording materials, pressure-sensitive copy paper, thermal copy paper, etc., in which the surface of an elongated support web that is running continuously at high speed is electrified before a coating solution such as a photographic emulsion or a suspension of magnetic particles is applied to the web.
In the manufacture of photographic materials, magnetic recording materials, recording paper, etc., coating methods are widely known in which the surface of a web running continuously at high speed is electrified before a coating solution is applied. Three typical examples of such methods are as follows:
(1) A discharge treatment is performed only at the start of coating application and/or at each time of the passage of web seams. (See Unexamined Published Japanese Patent Application No. 142565/1980).
(2) A potential of at least 0.1 kilovolt is applied to the area of the web where a bead of the coating solution is formed, or on the surface of the web immediately preceding that area. (See Unexamined Published Japanese Patent Application No. 146369/1986).
(3) Prior to application, the web is electrified to a constant charge potential under a degree of vacuum lower than a steady-state level, and, after application, the degree of vacuum is held above the steady-state level for a predetermined time before it is adjusted to the steady-state level. (See Unexamined Published Japanese Patent Application No. 258772/1989).
However, those methods have their own advantages and disadvantages. The first method is effective for the purpose of preventing the occurrence of undesirably thick coatings and streak defects at the start of application and at each time of the passage of web seams. However, it is useless for the purpose of achieving high-speed coating in a steady-state operation. If a voltage sufficient to create electric discharge were to be applied during steady-state coating operations, repellency defects tend to occur. The second method is effective for the purpose of preventing the occurrence of repellency defects during steady-state coating operations. However, if the necessary large quantity of electric charge is applied at the start of application or to nonsteady-state areas such as web seams, streaking and repellency defects are very likely to occur in steady-state areas. Conversely, if the applied electric field is small enough to avoid the occurrence of streaking and repellency defects in steady-state areas, nonsteady-state areas cannot be rendered completely stable. In the third method, in order to insure that electrification is performed at the constant charge potential reached in the steady-state operation, uniformity at the time of application and at each time of the passage of web seams is achieved by maintaining a degree of vacuum that is higher than the steady-state level.
The present invention still further relates to a coating method for use in the manufacture of photographic materials such as photographic films and print paper, materials for use in photomechanical processes, magnetic recording materials, pressure-sensitive copy paper, thermal copy paper, etc., in which a coating solution such as a photographic emulsion or a suspension of magnetic particles is applied to a continuously running elongated web, which method is particularly adapted for high-speed application.
Conventional methods for achieving high-speed application of coating solutions onto a continuously running web are classified into the following two major categories.
(1) A suction box divided into three compartments is provided in a hopper on the side where a web to be coated enters, with the three compartments aligned along the web, and a fluid, such as water, charged into the center compartment is evaporated so that the resulting vapor or air containing a large amount of water vapor is allowed to pass rapidly through the gap between the center compartment and the web (Unexamined Published Japanese Patent Application No. 32923/1973); a spray solution atomized by ultrasonic vibrations is sprayed so that it is deposited on the surface of the web on which the coating operation is to be performed (Unexamined Published Japanese Patent Application No. 31727/1987); or after preliminary treatment for rendering the web surface hydrophilic, the surface is moistened, and before it dries completely, a coating solution is applied ( Unexamined Published Japanese Patent Application No. 104376/1989).
(2) A coating solution is applied to the web after its entire surface has been electrified (U.S. Pat. No. 4,457,256).
However, the methods described above have their own defects. In the methods of the first category, the fluid evaporated in the center compartment of the suction box or the atomized fluid tends to condense around the web, or coarse liquid droplets that form directly can be deposited on the web to cause coating defects. In the second method, it is difficult in practice to form a uniform charge layer over the entire surface of the web, and the resulting unevenness in charging can potentially lead to uneven coating.